1. Field of the Invention
The present invention relates generally to audio processing and, more particularly, to an apparatus and method for eliminating pop noises that are generated when playing conditions are changed during the playing of audio.
2. Description of the Related Art
FIG. 1 illustrates the construction of a circuit for eliminating pop noises in conventional audio equipment.
The pop noise elimination circuit, as illustrated in FIG. 1, includes a power signal unit 100 for detecting the application and cutoff of power Vcc and generating a noise elimination drive signal for a predetermined period, a switching signal unit 102 for outputting a noise elimination drive signal in response to a signal that is generated when a mode or source is switched, a pop noise elimination unit 104 for receiving the noise elimination drive signal from the power signal unit 100 or switching signal unit 102 and grounding the output terminal of an audio input unit 103, and the audio output unit 103 connected to an audio output device AO.
The power signal unit 100 includes a condenser C2 configured to receive power Vcc via a resistor R1 and be charged with the power Vcc, a diode D1 connected in parallel to the resistor R1 in the reverse direction, a condenser C3 configured to receive the power Vcc via a diode D2 and be charged with the power Vcc, a transistor TR1 configured such that the charge voltage of the condenser C2 is applied to the base thereof, the charge voltage of the condenser C3 is applied to the emitter thereof and, thus, a noise elimination drive signal is output via the collector thereof, a ground resistor R3 connected to the collector of the transistor TR1, and a diode D3 configured to output a signal from a node N3 of the collector of the transistor TR1 and the ground resistor R3. The resistance value of the resistor R1 is set well above the resistance value of the resistor R2.
The switching signal unit 102 is configured to receive a switching signal via a General Purpose Input/Output (GPIO) control terminal and output a noise elimination drive signal via a resistor R4 and a diode D4.
The audio output unit 103 is configured to output an audio signal, which is received from an audio signal input device AI, to the audio output device AO via a buffer B1 and a Direct Current (DC) coupling condenser C1.
The pop noise elimination unit 104 is formed of a transistor TR2 that receives a noise elimination drive signal from the power signal unit 100 and/or the switching signal unit 102 and grounds the output terminal of the audio output unit 103 connected to the input terminal of the audio output device AO.
The operation of the pop noise elimination circuit of FIG. 1 is described below.
When power Vcc is applied, the power Vcc charges the condenser C2 via the resistor R1, which has a high resistance value, so that the charge voltage of the node 1 N1 slowly increases. In contrast, the power Vcc charges the condenser C3 via the diode D2 and the resistor R2, which has a small resistance value, so that the charge voltage of node 2 N2 rapidly increases.
The transistor TR1 is electrically conductive for a predetermined period corresponding to a charge time-constant period, after which the charge voltage of the condenser C2 reaches a predetermined level, so that a noise elimination drive signal is output through the collector thereof to a node 3 N3. Since the noise elimination drive signal is applied to the base of the transistor TR2 and makes the transistor TR2 conductive, the audio output device AO is grounded via the transistor TR2, therefore pop noises generated while the power Vcc is applied are eliminated.
When the voltage of the condenser C2 reaches a predetermined level after a predetermined period, the transistor TR1 becomes closed, so that a low potential signal based on a ground potential is output from the node 3 N3, that is, the collector of the transistor TR1. Accordingly, the transistor TR2 is closed, so that audio signals from the audio input device AI are normally output to the audio output device AO.
Meanwhile, when the power Vcc is cut off, the charge voltage of the condenser C2 is rapidly discharged through the diode D1, and the node 1 N1 enters a low state. Since the voltage of the condenser C3 is not discharged by the diode D2 connected in the reverse direction, the transistor TR1 becomes conductive.
As a result, the charge voltage of the condenser C3 is discharged through the transistor TR1 and the resistor R3, so that a high potential signal, that is, a noise elimination drive signal, is output from the node 3 N3 for a predetermined period corresponding to a discharge time-constant period, and the high-potential noise elimination drive signal makes the transistor TR2 conductive through the diode D3, therefore pop noises generated while the power Vcc is cut off are eliminated.
Meanwhile, when a mode or source is switched, high-potential switching signal is applied for a predetermined period via the switching signal input terminal of the switching signal unit 102.
The node 4 N4 is at a high state for the predetermined period and the transistor TR2 becomes conductive, so that the audio output device AO is grounded for the predetermined period, therefore pop noises generated while a mode or source is switched are eliminated.
The conventional pop noise elimination apparatus has both the problem of having a large system size, and the problem of the degradation of trustworthiness of a digital amplifier due to the repetition of high-speed switching.